A serial bus system in which frames are sent from a central station to a plurality of substations over a first conductor and information is sent from the substation to the central station over a second conductor. Each transmitted frame contains bits identifying whether it is for address, data or a command. An addressed substation sends a handshake back to the central station by changing the logic level of the second conductor. An address frame contains a bit indicating whether a message is to be written into a register of a substation or read from a register therein. Any substation can interrupt to request service from the central station by changing the logic level of the second conductor if the last command on the first conductor is an unaddress command indicating the end of a message.